This invention relates to an improved process for the manufacture of thianthrene compounds. Thianthrene and substituted compounds are useful in a variety of chemical processes, for example, as chemical intermediates in the preparation of dyestuffs such as thianthrene vat dyes. In addition, various thianthrene compounds are useful plasticizers, pesticides, and flameproofing agents. Recently it has been found that thianthrene compounds having electron withdrawing substitutents such as halogens, on the aromatic nuclei thereof are especially useful as components of catalyst systems for the directed nuclear chlorination of alkylbenzenes. Although thianthrene compounds have been known for many years, little effort has been directed toward the development of a process suitable for commercial manufacture.
One method described in the prior art (J.A.C.S., Vol. 78, 2163-2164, 1956) comprises reacting sulfur monochloride with refluxing benzene in the presence of aluminum chloride. The reaction product is then treated with iced hydrochloric acid and steam distilled to remove benzene and steam volatile impurities. The aqueous layer is then decanted off, and the residue filtered. The product is warmed with a sodium hydroxide solution, filtered and washed successively with water and ethanol. The residue is refluxed with an excess of glacial acetic acid, cooled, filtered and washed with ethanol to give 86% yield of crude brown thianthrene. The purity of the product may be improved (with a consequent decrease in yield) by additional distillation and recrystallization from glacial acetic acid.
While this laboratory scale method produces thianthrene, many disadvantages prevent its adoption for large scale commercial production. The method involves numerous unit operations to make pure thianthrene (no data is indicated in the literature on the percent purity of thianthrene attainable). Additional drawbacks to the adaptation of such a method to commercial production include the use of substantial quantities of extremely corrosive glacial acetic acid. Furthermore the thianthrene produced remains contaminated and occluded with by-product sulfur and diphenyl sulfide.
In a more recent development, it has been disclosed (U.S. Pat. No. 3,997,560, Dec. 14, 1976; and U.S. Pat. application Ser. No. 137,320, filed Nov. 1, 1976) that thianthrene compounds may be prepared by adding sulfur monochloride to an excess of a benzene compound in the presence of aluminum chloride, and reacting to form a thianthrene compound as an insoluble aluminum chloride complex; slurrying the complex in an inert inorganic liquid; treating the slurry with a Lewis base, such as ammonia pyridine, dimethylamine or the like, to free the thianthrene compound from the complex, and dissolving the thianthrene in an organic solvent therefor. Although the process disclosed is useful for the preparation of thianthrene compounds and adaptable to commercial production thereof, large scale operation presents certain difficulties, including for example, the use of large amounts of ammonia, and the noxious vapors thereof, and the need to further recover the desired thianthrene compound from solution in the organic solvent.
It is an object of the present invention to provide a process for the production and recovery of thianthrene based products which is direct and simple and which is adaptable to large scale commercial operations. A further object is to provide an improved method whereby a solid high purity thianthrene product may be produced in high yields. A still further object is to provide an improved process for the recovery of thianthrene compounds from a thianthrene compound-aluminum chloride complex.